The aim of this project is to identify and study the structural characteristics of prolactin which are responsible for its biological activity. Assays to be used in estimating activity are the systemic pigeon crop-sac assay, induction of N-acetyl lactosamine synthetase in cultured mammary tissue of pregnant mice, and a radioligand assay to study binding of prolactin to membranes. Several methods, chemical and enzymatic, of altering the structure of prolactin are planned. In each case, whether an altered, intact molecule or a fragmented molecule, the product(s) will be chemically characterized and tested in the available assay systems. Comparisons will be made with human placental lactogen and human growth hormone, each of which has prolactin activity and shares some structural features with prolactin. Also, tests will be made to determine whether inactive products compete with or inhibit native prolactin. In addition, the cultured mammary tissue assay will be modified to the use of human and monkey tissue, as specimens become available, to determine whether species differences exist in prolactin activity.